Actually, as someone who has played Traveller from its very early years (starting in 1979), I can tell you that the Third Imperium game setting was not part of the original set. As a matter of fact, there's no default setting in the basic Little Black Book boxed set. It wasn't until later, when lots of fans of the game system started bugging GDW about a setting, that they developed the whole setting that we know of today. You can ask Loren Wiseman over at Steve Jackson Games all about this. He was with GDW back when all this was happening.